In some demyelinating diseases of man, typified by multiple sclerosis (MS) the condition is frequently not determined until autopsy. This silent MS, which does not reflect the significant demyelination in the CNS, is worthy of investigation of the factors responsible for clinical protection in the face of putative severe immunological attack. The strain 2 guinea pig provides an age-, sex- and strain - dependent model for this situation in that female but not male juvenile animals sensitized with spinal cord antigens develop clinical experimental allergic encephalomyelitis (EAE), an established immunological animal model of MS, but both male and female guinea piglets show striking histological change and demyelination. Evidently, male juveniles are competent immunologically, but factors intervene to prevent the clinical course. Neither male nor female strain 2 juveniles show the cortical cataract of acute EAE found in this laboratory in strain 13's, but 13/2 hybrids which in contrast to strain 2 are susceptible to acute EAE, do manifest cataracts. An autosomal dominant nuclear cataract of strain 13 is being investigated for its particular crystallin production, genetics, phylogeny, histology, and biochemical and serological characteristics.